Many devices exist for performing an operation on a moving web wherein a gas or gas pressure differential is applied. For example, in the art of papermaking, a wet or embryonic paper web can be partially dried or dewatered by means of an applied gas pressure differential using nozzles, a compressive roll with holes or grooves for pressurized air, an "air press" or other devices wherein a gas pressure differential forces air into a web to displace liquid water and/or to remove water by evaporation. Many prior systems, particularly those having compressive rolls, impose high compressive force on the web and are not suitable for many low density materials such as tissue. Systems of nozzles are typically inadequate due to the low residence time provided for air penetration into the web. Nozzle systems also either suffer from high leakage if the nozzle is not in contact with the web or from high fabric wear if the fabric wraps the surfaces of the nozzle to provide some degree of sealing. A fabric wrapping a nozzle with a small effective radius of curvature is particularly likely to experience wear problems.
Flat pressurized boxes, such as steam boxes, while capable of good residence time in some cases, suffer from high leakage from the sides of the steam chamber. Steam boxes for heating paper webs in particular have tended to be flat boxes with a finite gap between the web and the sides of the box. Such gaps or clearances allow significant volumes of air to enter, in part due to the air boundary layer traveling with the web. Intentionally bleeding steam to oppose the boundary layer or using a steam curtain to prevent entry of the boundary layer is inherently inefficient.
Recognizing the difficulty of providing adequate seals in pressurized paper drying chambers, some have proposed the use of high-velocity heated air impingement that relies on the momentum of the air to push through the web for paper drying without attempting to use seals. This technique is intended to minimize lateral migration of the drying air along the surface of the web, thereby reducing the need for sealing. Even if this method reduces the lateral flow of air, the extent of treatment is limited by the brief contact time of the pressurized gas with the web due to the narrow jets employed. Without suitable seal elements, leakage still will not be prevented.
Rotary devices, such as cylindrical through dryers and suction rolls, can be operated to pass air through a fibrous sheet but are complex and costly devices. Further, the surface of the rotary device or other supporting surfaces in contact with the web have significant closed areas where gas flow is blocked, resulting in nonuniform penetration of the gas through the web.
An excellent system for effective gas treatment of a moving web is the dewatering system disclosed in commonly owned copending application Ser. No. 08/961,915 of Hada et al. filed Oct. 31, 1997 and incorporated herein by reference in its entirety. This application teaches an air press wherein a non-rotating upper plenum is used to apply pressurized air to a moist paper web while the web is sandwiched between two pervious fabrics. The pressurized plenum cooperates with a lower vacuum box on the opposite side of the sandwiched paper web such that the overall pressure differential across the web is greater than if the pressurized plenum were used alone at a predetermined pressure. An important issue in the operation of an air press is preventing unnecessary leakage of gas out of the plenum into the surrounding atmosphere. Hada et al. disclose a set of cross-direction seals (seals running in the cross-direction) for the leading and trailing edges of the plenum (the leading edge being closest to the headbox) and a pair of machine-direction seals running in the machine direction to seal the side edges of the plenum. Hada et al. also disclose a lever system for increasing sealing force on the seals responsive to measurement of air pressure in the plenum. The principle of operation is that excessive leakage will result in a reduction of pressure in the plenum, which can then be compensated by increasing the applied pressure to the systems of seals.
Though capable of opposing leakage and preventing large leaks, such a system may lead to excess fabric wear, particularly in wide machines, because sealing is generally performed across the entire width of the machine, rather than solely in the locations where leak reduction is needed. Further, use of internal pressure measurements to detect leak control may lead to some false readings of leakage when pressure fluctuates for other reasons, such as changes in web properties or compressor operation or air temperature. Further still, a single macroscopic measure of pressure cannot be used to locate specific leaks, only leakage in general. Therefore, what is lacking and needed is a method and apparatus for detecting the specific location of leaks and applying corrective remedies to prevent leakage only at the locations where leakage is occurring.